Impact on key nutrients carbohydrates

Reducing sugars such as glucose and lactose participate in Maillard reactions, which will be discussed further in section 14.3. The shear forces during extrusion can also create reducing sugars from complex carbohydrates as well as from sucrose and other sugars. Sucrose losses of up to 20% were found in protein-enriched biscuits (Noguchi and Cheftel, 1983). While sucrose loss may affect product color and flavor, there is an opportunity to reduce the content of indigestible oligosaccharides that can cause flatulence. Sucrose, raffinose and stachyose decreased significantly in extruded pinto bean high-starch fractions (Borejszo and Khan, 1992). Corn-soy snacks had lower levels of both stachyose and raffinose compared to unextruded soy grits and flour, but values were not corrected for the 50-60% corn present (Omueti and Morton, 1996). Starch and stachyose were lower in extruded peas compared to raw peas (Alonso et al, 2000), but an increase in total free sugars did not fully account for these losses (Fig. 14.2).

  1. 14.2 Carbohydrate changes (g/kgdry matter) due to extrusion of peas (Pisum sativum L) at an exit temperature of 145 °C and 25% feed moisture. (Adapted from Alonso et al, 2000)
  2. 14.2 Carbohydrate changes (g/kgdry matter) due to extrusion of peas (Pisum sativum L) at an exit temperature of 145 °C and 25% feed moisture. (Adapted from Alonso et al, 2000)

Starch is usually the major food constituent in extruded foods such as breakfast cereals, snacks and weaning foods. Humans and other monogastric species do not readily digest native or ungelatinised starch. Unlike many thermal processes, extrusion cooking gelatinises starch at fairly low (12-22%) moisture levels. Removal of cooking water is not a problem, and leaching of water-soluble nutrients is avoided. Increased temperature, shear, and pressure during extrusion increase the rate of gelatinisation, but lipids, sucrose, dietary fiber and salts can retard gelatinisation (Jin et al, 1994). While full gelatinisation may not occur during extrusion, digestibility is often improved (Wang, S et al, 1993).

During extrusion, starch molecules can be physically broken into smaller, more digestible fragments. For example, amylopectin branches can be sheared off the main molecule, with larger molecules experiencing the greatest effect (Politz et al, 1994b). Both amylose and amylopectin molecules may be affected, however. Molecular weight in extruded wheat starch was retained better under processing conditions of higher die temperature (185 °C) and feed moisture (20%) (Politz et al, 1994a). Screw configurations using more reverse and high-shear elements favor starch breakdown (Gautam and Choudhoury, 1999).

Lower molecular weight starch fragments may be sticky, thereby increasing the risk for dental caries, since bacteria in the mouth rapidly ferment these dextrins. Toothpack, the amount of material retained on teeth, has been used as an indication of the severity of extrusion processing. Bjorck and co-workers (1984) found that white wheat flour extruded under 'mild' and 'severe' conditions caused drops in dental plaque pH comparable to those obtained with glucose.

While easily-digested starch is desirable for infants and invalids, the resulting rapid post-prandial rise in blood sugar and insulin levels is thought to be a risk factor for development of insulin insensitivity and Type II, or adult-onset, diabetes. Extrusion offers the ability to reduce the high glycemic index (GI) of some foods by converting starch to digestion-resistant starch (RS). Theander and Westerlund (1987) reported transglycosidation in extruded wheat flour, presumably from attachment of sheared amylopectin branches to other reactive sites. The resulting novel bonds would be resistant to digestion by enzymes. Addition of high amylose starch also reduces digestibility. As much as 30% resistant starch was reported when high amylose starch was reacted with pullulanase prior to extrusion (Chiu et al, 1994). Extruded high amylose rice noodles had lower starch digestibility and reduced GI (Panlasigui et al, 1992).

An evolving area of research involves the use of additives to promote RS formation. Adding 30% corn, potato or wheat starch did not increase RS values in cornmeal, but RS and fiber values more than doubled when 7.5% citric acid was used, and 30% high-amylose cornstarch with 5 or 7.5% citric acid resulted in values of 14%, compared with slightly more than 2% in 100% cornmeal (Unlu and Faller, 1998). Polydextrose may have been formed during extrusion. Limitations to this approach would be the expenses of the additives and sour taste of the extrudates. Yields of up to 93.7% oligosaccharides and polydextrose were reported when glucose-citric acid mixtures were extruded at different barrel temperatures (Hwang et al, 1998).

Longer cellulose fibers added to cornstarch decreased starch solubility (Chinnaswamy and Hanna, 1991). Removal of insoluble dietary fiber from wheat flour in combination with 20% protein addition resulted in pasta with significantly delayed dextrin release under in vitro digestion conditions (Fardet et al, 1999), possibly due to enhanced protein-starch interactions. Amylose forms complexes with lipids during extrusion, thereby reducing both starch and lipid availability. This phenomenon will be addressed in section 14.4.

The term dietary fiber is used to describe nondigestible carbohydrates and associated compounds such as lignin. Although a global definition of dietary fiber does not yet exist, there is a consensus that adequate fiber consumption is essential for good health. Analytical methods for quantitating dietary fiber vary considerably. The AOAC total dietary fiber method used for US nutritional labeling does not measure compounds that are soluble in 80% aqueous ethanol such as certain fructans and polydextrose, and this procedure does not detect changes in extruded fiber solubility. If different dietary fiber fractions are not analysed separately, it is possible to overlook important changes in dietary fiber composition and functionality caused by extrusion.

Like starch, branched dietary fiber molecules are susceptible to shear. The smaller fragments may be soluble in water. Fragments may also combine to form large insoluble complexes that may be analysed as lignin. Although extrusion did not affect pectin, both soluble and insoluble nonstarch polysaccharides (NSP) were increased in extruded oatmeal and potato peels (Camire and Flint, 1991). Corn meal fiber was unaffected by extrusion under the same conditions as the other foods. Extruded beans (Phaseolus vulgaris L) had total fiber values comparable to those before extrusion, but a redistribution of insoluble to soluble fiber occurred (Martín-Cabrejas et al, 1999). Sugar beet pectin and hemicellulose molecular weight decreased with extrusion, and water solubility of those compounds increased by 16.6 to 47.5% (Ralet et al, 1991). Extrusion increased the solubility of beta-glucans in regular and waxy barley cultivars (Gaosong and Vasanthan, 2000).

Does the 'soluble' fiber created during extrusion have the same health benefits as natural forms such as pectin and b-glucan? Viscous gels formed in the small intestine trap bile acids and thus may contribute to lower serum cholesterol levels; the soluble fiber matrix is also thought to slow glucose absorption from the small intestine. Extrusion increased the viscosity of aqueous suspensions of wheat, oats and barley (Wang and Klopfenstein, 1993). Although increased in vitro viscosity was correlated with higher levels of soluble citrus peel fiber after extrusion (Gourgue et al, 1994), in vitro starch digestion and glucose diffusion were unaffected. Extrusion of wheat flakes containing guar gum did not reduce the guar gum's ability to lower post-prandial blood glucose and insulin in healthy adults (Fairchild et al, 1996). In an intervention study involving middle-aged men with hyperlipidemia, baked goods fortified with 92g/day extruded dry white beans did not lower serum lipoproteins (Oosthuizen et al, 2000).

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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